The present invention pertains to the electrical power supply art and, more particularly, to improved power supply shutdown apparatus.
Power supply shutdown circuits are well known, particularly in the electronics art. Such shutdown circuits sense for the condition of any defined fault, such as overcurrent, overvoltage, undervoltage, extreme operation temperature and so on and inhibit operation of the supply in response to the fault occurring.
In a particular application, a power shutdown supply which responds to a low voltage condition is required for use in a mobile data terminal. There, proper functioning of the terminal requires that a specified voltage be applied to a central microprocessor. Should the power supply voltage fall below a given value, the microprocessor is likely to generate numerous errors.
For the above mentioned mobile data terminal application, a switching mode type power supply is used. In such supplies, a controlled oscillator drives the primary of a transformer. Power supply voltage is taken from the transformer's secondary, after suitable rectification and filtering. A feedback signal from the supply's output is compared with a stable DC reference to generate an error signal which is coupled to the controllable oscillator. The error signal causes either the frequency or duty cycle, depending upon the oscillator's design, to vary in such a manner that the output voltage is maintained at a desired level.
In power supply low voltage shutdown circuits according to the prior art, the output voltage from the supply is compared to a fixed stable reference and the supply inhibited if the output voltage has a predetermined relationship to the reference. The problem with such prior art shutdown systems is not only do they require a temperature and time stable voltage reference, but also they require temperature and time stable associated components.This results in a shutdown circuit of considerable cost. Thus, the prior art has felt a need for a precise yet inexpensive low voltage shutdown circuit, particularly suited for switching mode type power supplies.